Linux: use gcc to compile c++ files

Please see also bug 4665166. Note we have already fixed that one for the HotSpot
workspace, this bug is filed for J2SE workspace, as apparently both
libfontmanager.so and plugin still have dependency on libstdc++-libc6.1-1.so.2
in b15. That could cause library-not-found errors on new Linux versions
which do not have the ancient gcc-2.91 installed. The solution for now is to
statically link libstdc++. And that's extremely easy to do and risk-free, just
use the gcc driver instead of "g++" to compile and link C++ programs.

The only change needed is in make/common/Defs-linux.gmk,

34c34
< CXX = $(COMPILER_PATH)gcc
---
> CXX = $(COMPILER_PATH)g++

It's been used in Blackdown's JDK port for quite a while and has been
proved can solve many of the compatibility issues.

I am logging the bug as P1, as it's expected major Linux vendors will be
shipping their next release later this year based on gcc-3.x (e.g. Slackware 8.1
is just released this week). We need to prepare for that, otherwise we could
see a flurry of bugs on JDK not working with new Linux releases.